1. Field of the Invention
The invention relates to an electric incandescent lamp comprising
a translucent lamp vessel with an axis provided with a reflective coating, PA1 a first and a second wound filament respectively having a first and a second end portion and which are arranged in the lamp vessel near the axis thereof, and PA1 current supply conductors respectively connected to the end portion of the first and the second filament and which exit from the lamp vessel.
2. Description of the Related Art
Such a lamp is known from Netherlands Patent 121 505.
In the known lamp, there are two linear filaments which are arranged at an angle of 45.degree. to the axis and either at right angles or parallel to each other.
This arrangement results in that portions of both filaments are at a great distance from the axis of the lamp vessel. This has the consequence that the light beam formed by the reflective coating of the lamp vessel has a large width and consequently a low luminous intensity at the centre thereof. Another consequence is that the light beam is inhomogeneous and not axially-symmetrical. The filaments may have different powers; for example, one filament may have a power twice that of the other.
Various other incandescent lamps having several parallel-connected filaments are known. For example, Patent EP 0 280 475-A discloses a lamp having two transversely arranged filaments forming a closed figure. One of these filaments is designed for a longer life than the other. This has for its object that the lamp will continue to emit light for a long time after the other filament has already burnt through.
Such a lamp is not efficient because the filament having the longest life inevitably has a lower light output per unit of energy consumed. The net light output of the filament having the longest life is even lower than the gross light output thereof because by evaporation of the material of the filament having the shortest life, after it has burnt through, a light-absorbing deposit of evaporated filament material forms on the lamp vessel.
A similar lamp is known from Patent Specification GB 426 477.
The aforementioned Patent EP 0 280 475-A also discloses a lamp having three filaments. Two of these filaments are connected in parallel with each other and both in series with the third filament. These filaments are designed to have different lives; one of the parallel-connected filaments has the shortest life, while the third filament has the longest life.
In incandescent lamps having a reflective coating, the filament is usually arranged transversely to the axis of the lamp and is helically wound around its own axis. Such an incandescent lamp is known, for example, from Applicant's U.S. Pat. No. 4,788,469. Due to the comparatively great length of an incandescent filament for operation at mains voltage, this filament extends, helically around its axis, to a comparatively great distance from the lamp axis. A comparatively large width of the light beam formed is the result. Other consequences of the position and the shape of the filament are the formation of a non-symmetrical light beam and a comparatively low luminous intensity at the centre thereof.